Fine coals (having a particle size of less than about 250 microns, and typically less than about 150 microns, are a problem in most coal recovery operations. It is extremely difficult to dewater or remove liquid from fine coal slurries. A disproportionate amount of the moisture in minus 28 mesh.times.0 filter cake (less than 600 microns) is associated with the minus 100 mesh (150 microns or less) fraction.
The -28 mesh.times.100 mesh fraction is easily separable from fine coal. Numerous technologies exist for separating and dewatering this size fraction including hydrocyclones, sieve bends and screen-bowl centrifuges.
The -100 mesh fraction has a large surface area to mass ratio or surface area to volume ratio. A filter cake of 60-65 weight percent coal is often the maximum concentration achievable. The handling of such wet coal fines is expensive and difficult. Such materials present a disposal problem, and can shorten the economic life of many mines. At many plants, the fines fraction is dewatered to the greatest extent possible and blended in with the run-of-mine coal. This is done because a) the coal is too valuable to throw away, and b) disposing of fines in silt ponds represents an increasing environmental liability. If the wet fines could be removed from the product without disposal in a settling pond, the value of the mine or preparation plant's product would be improved.
Coal fines from coals with high ash (minerals) content are typically treated by froth flotation to separate the minerals, resulting in a floated fine coal fraction and a fine waste slurry fraction which generally contains fine coal as well as ash particles. The processes of this invention are suitable for concentrating both fractions. Fuel value can be recouped from the floated fine coal fraction, and the ash-containing fraction can be consolidated for more economical disposal.
Electric power utilities using boiler operations require modulation of fuel supply depending upon consumer demand. Modulating control of the flame with pulverized coal is very problematic. Delivery of liquid or gaseous fuels to the combustion chamber provides much more reliable operation. Therefore, in the eastern United States, base-load power is generally supplied through pulverized coal combustion while peak demand is satisfied by oil or gas. However, the cost of these fuels can be three to five times the cost of coal on a per-million-Btu basis.
It has been estimated that in the coal-producing regions of the U.S. alone, there exist between 10 and 50 billion tons of fine coal impounded in settling ponds ("gob piles") at mines which have been closed. Much of this coal is of high quality. Some of it would require some form of beneficiation. Nevertheless, the ponds represent both a huge resource as well as a substantial environmental liability. The technologies to enable utilization of this resource have been limited.
U.S. Pat. No. 4,952,317, incorporated herein by reference, describes horizontally oscillated liquid-permeable membranes useful for removing selected components from colloidal suspension. The invention is disclosed as being useful for separating of particles below 50 microns such as finely divided clays, protein molecules and ions. In practice, the device has generally been used with solids in the sub-10 micrometer range such as pigments, muds, contaminated oils and magnetic media materials.
A method for concentrating coal fines to produce a valuable product in mining and preparation plant operations, and for recovery of coal fines from silt ponds is therefore desirable. Additionally, a method for concentrating high-mineral content fine coal slurries for more economic disposal is desirable.
Prior art patents dealing with fine coal slurries include U.S. Pat. No. 4,593,859 involving preparation of a deashed high solid concentration coal slurry; U.S. Pat. No. 4,765,781 dealing with pipeline transportation of fine coal slurries; U.S. Pat. Nos. 4,810,259, 4,645,514, 4,634,451 and 4,687,490 dealing with methods to improve viscosity and other properties of fine coal slurries; and U.S. Statutory Invention Disclosure 02170914 dealing with the use of dispersants in coal slurries.